ZrO2 based ceramic material and method of producing the same

ABSTRACT

A ZrO 2  based ceramic material having excellent mechanical strength and fracture toughness comprises a first phase of ZrO 2  grains containing CeO 2  as a stabilizer and having an average grain size of 5 μm or less, a second phase of Al 2  O 3  grains having an average grain size of 2 μm or less, and a third phase of elongated crystals of a complex oxide of Al, Ce, and one of Mg and Ca. At least 90 vol % of the first phase is composed of tetragonal ZrO 2 . An aluminum (Al) content in the ceramic material is determined such that when Al of the complex oxide is converted to Al 2  O 3 , a total amount of Al 2  O 3  in the ceramic material is within a range of 0.5 to 50 vol %. A content of the third phase in the ceramic material is determined within a range of 0.5 to 5 by area %. It is preferred that fine Al 2  O 3  grains having an average grain size of 1 μm or less of the second phase are dispersed within the ZrO 2  grains at a dispersion ratio of at least 2%.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a ZrO₂ based ceramic material havingexcellent mechanical strength and toughness, and a method of producingthe same.

2. Disclosure of the Prior Art

Owing to superior heat-resisting property, wear-resistance, andcorrosion-resistance of ceramics such as alumina (Al₂ O₃), zirconia(ZrO₂), silicon nitride (Si₃ N₄), or the like, it is expected to use theceramics for wide applications as a turbocharger rotor for an automobileengine, various kinds of edged tools, mechanical parts such as abearing, a mechanical seal and the like, a cutting bite, a drillingtool, crushing media, an optical connector ferrule, a dice, a saw and soon. However, mechanical strength and toughness of the ceramics are notalways sufficient for those applications. That is, since the ceramicsusually show very poor plastic deformation unlike a metal material,macro cracks tends to rapidly and readily proceed from fine defects orflaws in the ceramics. Therefore, it is desired to develop a ceramicmaterial having improved mechanical strength and toughness, which can beused safely for a longer time period in those applications. As anexample, ceramic materials comprising a CeO₂ -partially stabilized ZrO₂and Al₂ O₃ are being studied.

Japanese Patent Publication KOKOKU! No. 64-7029 discloses a ceramicmaterial comprising 61 to 87 wt % of ZrO₂, 11 to 27 wt % of CeO₂ (ceriumdioxide), and 20 wt % or less of Al₂ O₃. CeO₂ forms a solid solutionwith ZrO₂, so that ZrO₂ crystals of the ceramic material are composed of20% or less of monoclinic ZrO₂ and/or cubic ZrO₂ and the balance oftetragonal ZrO₂. This prior art discloses that when the content of Al₂O₃ is more than 20 wt %, a sintering temperature of the ceramic materialincreases, so that a grain growth of the zirconia crystals is caused.This will bring degradation in the mechanical strength of the ceramicmaterial due to an enlargement of a flaw size.

Japanese Patent Early Publication KOKAI! No. 5-246760 discloses a ZrO₂based ceramic material comprising a matrix of a partially stabilizedZrO₂ containing 5 to 30 mol % of CeO₂ and a secondary phase of at leastone selected from Al₂ O₃, SiC, Si₃ N₄, B₄ C, carbides, nitrides andborides of elements of groups IVa, Va, VIa of the periodic table. Finegrains of the secondary phase are dispersed within grains as well asgrain boundaries of the ZrO₂ matrix. When the content of CeO₂ is morethan 30 mol %, the mechanical strength of the ceramic material lowersdue to an increase of cubic ZrO₂. When the content of CeO₂ is less than5 mol %, a formation of metastable tetragonal ZrO₂ is not sufficient.The ceramic material contains 0.5 to 50 vol % and more preferably 2.5 to30 vol % of the secondary phase.

Japanese Patent Early Publication KOKAI! No. 8-268755 discloses a ZrO₂based ceramic material consisting essentially of 0.5 to 50 vol % of Al₂O₃ having an average grain size of 2 μm or less and the balance of apartially stabilized ZrO₂ having an average grain size of 5 μm or less.The partially stabilized ZrO₂ consists essentially of 8 to 12 mol % ofCeO₂, 0.05 to 4 mol % of TiO₂ and the balance of ZrO₂. Fine Al₂ O₃grains having an average grain size of 1 μm or less are dispersed withinthe ZrO₂ grains at a dispersion ratio of at least 2%. The dispersionratio is defined as a ratio of the number of Al₂ O₃ grains dispersedwithin the ZrO₂ grains relative to the number of the entire Al₂ O₃grains dispersed in the ceramic material.

In addition, a ceramic material comprising 10 wt % of Al₂ O₃, 1.5 wt %of MnO and the balance of CeO₂ -partially stabilized ZrO₂ is disclosedin Journal of American Ceramic Society, 75 5! 1229-38 (1992). Thepartially stabilized ZrO₂ contains 12 mol % of CeO₂. This prior art alsodiscloses that MnO reacts with both CeO₂ and Al₂ O₃ during a sinteringstep to form a new phase having an approximate composition of CeMnAl₁₁O₁₉. The ceramic material exhibits a mechanical strength of 650 MPa infour-point bending and a fracture toughness of 7.6 to 10. 3 MPa.m^(1/2)in compact tension tests.

Thus, various attempts have been made to improve the mechanicalproperties of the ZrO₂ --Al₂ O₃ ceramic materials. However, there isroom for further improvement of the mechanical properties.

SUMMARY OF THE INVENTION

A primary object of the present invention is to provide a ZrO₂ basedceramic material having improved mechanical strength and toughness. Thatis, the ceramic material comprises a first phase of ZrO₂ grainscontaining CeO₂ as a stabilizer and having an average grain size of 5 μmor less, a second phase of Al₂ O₃ grains having an average grain size of2 μm or less, and a third phase of elongated crystals of a complex oxideof Al, Ce, and one of Mg (magnesium) and Ca (calcium). At least 90 vol %of the first phase is composed of tetragonal ZrO₂. An Al content in theceramic material is determined such that when Al of the complex oxide isconverted to Al₂ O₃, a total amount of Al₂ O₃ in the ceramic material iswithin a range of 0.5 to 50 vol %. A content of the third phase in theceramic material is determined within a range of 0.5 to 5 by area %.

It is preferred that fine Al₂ O₃ grains having an average grain size of1 μm or less of the second phase are dispersed within the ZrO₂ grains ata dispersion ratio of at least 2%. The dispersion ratio is defined as aratio of the number of the Al₂ O₃ grains dispersed within the grains ofthe ZrO₂ grains relative to the entire Al₂ O₃ grains dispersed in theceramic material.

It is also preferred that the elongated crystals has an average lengthof 2 to 50 μm with a maximum length up to 70 μm. In particular, it ispreferred that an average aspect ratio of the elongated crystals iswithin a range of 2 to 25. The aspect ratio is defined as a ratio oflength to width of each of the elongated crystals.

A further object of the present invention is to provide a method ofproducing the ZrO₂ based ceramic material of the present invention. Thatis, a first constituent corresponding to a composition of 8 to 12 mol %of CeO₂, 0.01 to 0.1 mol % of one of MgO (magnesium oxide) and CaO(calcium oxide), and the balance of ZrO₂ is mixed with a secondconstituent for forming Al₂ O₃, to obtain a mixed powder. The mixedpower is molded to a green compact having a desired shape. The greencompact is sintered in an oxidative atmosphere at a temperature between1400° C. and 1600° C. under an atmospheric pressure. A reaction of Ceand one of Mg and Ca supplied from the first constituent with Alsupplied from the second constituent in the oxidative atmosphere duringthe sintering form the third phase of the ceramic material.

These and still other objects and advantages features of the presentinvention will become more apparent from the following description andexamples of the present invention when taken in conjunction with theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a scanning electron micrograph of a ZrO₂ based ceramicmaterial of Example 5 of the present invention;

FIG. 2 is a scanning electron micrograph of an elongated crystal formedin the ceramic material;

FIG. 3 is a chart of an energy dispersive X-ray analysis (EDAX) of aZrO₂ grain of the ceramic material of Example 5;

FIG. 4 is a chart of the energy dispersive X-ray analysis (EDAX) of anAl₂ O₃ grain of the ceramic material;

FIG. 5 is a chart of the energy dispersive X-ray analysis (EDAX) of anelongated crystal of a complex oxide of the ceramic material;

FIG. 6 is a transmission electron micrograph of the ceramic material ofExample 5 ; and

FIG. 7 is a transmission electron micrograph of the ceramic material.

DETAIL DESCRIPTION OF THE INVENTION

A ZrO₂ based ceramic material of the present invention comprises a firstphase of ZrO₂ grains containing CeO₂ as a stabilizer and having anaverage grain size of 5 μm or less, a second phase of Al₂ O₃ grainshaving an average grain size of 2 μm or less, and a third phase ofelongated crystals of a complex oxide of Al, Ce, and one of Mg and Ca.

As to the first phase, CeO₂ forms a solid solution with ZrO₂ and behavesas the stabilizer for metastably keeping tetragonal ZrO₂, which is acrystal phase stable at a high temperature, down to a room temperature.In the present invention, the first phase contains CeO₂ in such anamount that at least 90 vol % of the first phase is composed oftetragonal ZrO₂. When the volume content of tetragonal ZrO₂ is less than90 vol %, there is a tendency that mechanical properties of the ceramicmaterial deteriorate because of an excess amount of monoclinic and/orcubic ZrO₂ in the first phase. In particular, when an excess amount ofmonoclinic ZrO₂ appears in the first phase, micro cracks often developin the ceramic material.

As to the second phase, an Al content in the ceramic material isdetermined such that when Al of the complex oxide is converted to Al₂O₃, a total amount of Al₂ O₃ in the ceramic material is within a rangeof 0.5 to 50 vol %, and more preferably 2.5 to 30 vol %. When the amountof Al₂ O₃ is less than 0.5 vol %, a contribution of the second phase tothe mechanical properties of the ceramic material is not obtained. Inaddition, a sufficient amount of the third phase is not formed in theceramic material. As the amount of Al₂ O₃ increases more than 50 vol %,the mechanical toughness of the ceramic material gradually lowers.

As to the third phase, a content of the third phase in the ceramicmaterial is determined within a range of 0.5 to 5 by area %. The content(area %) of the third phase is represented by the following equation;

    Content of Third Phase (area %)=(t/T)×100

where "T" is a total area of an observation region of the ceramicmaterial observed by the use of a scanning electron microscope (SEM)and/or transmission electron microscope (TEM), and "t" is an area of thethird phase exposed on the observation region of ceramic material. Whenthe content of the third phase is less than 0.5 area %, a contributionof the third phase to an improvement of mechanical toughness of theceramic material is not sufficiently obtained. When the content of thethird phase is more than 5 area %, there is a problem that variations inthe mechanical strength increase, and an average mechanical strength ofthe ceramic material decreases.

It is believed that mechanical properties of the present ceramicmaterial are improved according to the following mechanism. That is,residual stress fields are generated around the ZrO₂ grains, Al₂ O₃grains, and the elongated crystals of the complex oxide during a coolingstep from a sintering temperature of the ceramic material. At theresidual stress fields, a large number of dislocations occur within theZrO₂ grains. The dislocations are piled up each other to form sub-grainboundaries within the ZrO₂ grains. The formation of the sub-grainboundaries provides a fine grain structure, and is useful to increase acritical stress necessary for causing a stress induced phasetransformation from tetragonal ZrO₂ to monoclinic ZrO₂. In addition,since cracks developed in the ceramic material are bowed or deflected bythe Al₂ O₃ grains and the elongated crystals uniformly dispersed in thegrain boundaries of the ZrO₂ grains, further progresses of the crackswould be effectively prevented to improve the fracture toughness of theceramic material.

It is preferred that the elongated crystals have an average length of 2to 50 μm with a maximum length up to 70 μm. When the average length andthe maximum length of the elongated crystals are satisfied with theabove ranges, it is possible to provide the composite ceramic materialhaving a higher fracture toughness, while minimizing variations of themechanical strength of the ceramic material. By the way, a fracturestrength (σf) of a ceramic material can be expressed by Griffith'sequation:

    σf=(1/Y)×(KIC/c.sup.1/2)

where "Y" is a shape constant, "KIC" is a value of fracture toughness,"c" is a fracture-origin size (crack and/or defects of microstructure).For example, pure Al₂ O₃ ceramics usually show about 3 MPa.m^(1/2) of"KIC". It is presumed that an average fracture-origin size of the Al₂ O₃ceramics is about 4 μm. On the other hand, most of the ZrO₂ basedceramic materials of the present invention show about 12 MPa.m^(1/2) ormore of "KIC". When assuming that a pure Al₂ O₃ ceramic and a ZrO₂ basedceramic material of the present invention are of a same mechanicalstrength (σf) and a same shape constant (Y), and the "KIC" values of theAl₂ O₃ ceramic and the ZrO₂ based ceramic material are 3 MPa.m^(1/2) and12 MPa.m^(1/2), respectively, the Griffith's equation teaches that thefracture-origin size of the ZrO₂ based ceramic material is 64 μm.Therefore, when the maximum length of the elongated crystals in the ZrO₂based ceramic material is up to 64 μm, it is presumed that the thirdphase may not behave as the fracture origin. Results of the attachedExamples follow this presumption well. From observations of fractureorigins of the present ceramic materials, it is preferred that themaximum length of the elongated crystals is up to about 70 μm to preventthe behavior of the third phase as the fracture origin. In addition, itis preferred that an average aspect ratio of the elongated crystals iswithin a range of 2 to 25. The aspect ratio is defined as a ratio oflength to width of the elongated crystals.

It is also preferred that fine Al₂ O₃ grains having an average grainsize of 1 μm or less are dispersed within the ZrO₂ grains of the firstphase at a dispersion ratio of at least 2% to form a nano-compositestructure in the ceramic material. The dispersion ratio is defined as aratio of the number of Al₂ O₃ grains dispersed within the ZrO₂ grainsrelative to the number of the entire Al₂ O₃ grains dispersed in theceramic material. The nano-composite structure with the dispersion ratioof at least 2% further improves the mechanical properties of the ceramicmaterial. In addition, a residual stress field is generated around eachof the Al₂ O₃ grains dispersed within the ZrO₂ grains by a mismatch ofthermal expansion coefficients between ZrO₂ and Al₂ O₃, so that the ZrO₂grains can be remarkably reinforced. It is further preferred that fineZrO₂ grains having an average grain size of 1 μm or less are partlydispersed within the elongated crystals of the third phase and/or withinrelatively large Al₂ O₃ grains, in order to obtain a nano-compositestructure in the ceramic material. This nano-composite structure furtherimproves the mechanical properties of the ceramic material.

It is preferred that 0.05 to 4 mol % of TiO₂ is dissolved into the ZrO₂grains of the first phase. In a ZrO₂ --TiO₂ phase diagram, it is wellknown that tetragonal ZrO₂ forms a solid solution with up to about 18mol % of TiO₂ at a high temperature. TiO₂ is capable of keeping thetetragonal ZrO₂ metastably at a room temperature as well as Y₂ O₃ andCeO₂, and enhancing a grain growth of ZrO₂. Therefore, when an excessamount of TiO₂ is added to ZrO₂, the mechanical strength of the ceramicmaterial will decrease because of an abnormal grain growth of ZrO₂. Whenthe TiO₂ content is within the above range, it is possible to bringabout a controlled grain growth of the ZrO₂ grains to form thenano-composite structure explained above. That is, when the TiO₂ contentis less than 0.05 mol %, a required grain growth of ZrO₂ for forming thenano-composite structure is not achieved. When the TiO₂ content is morethan 4 mol %, the abnormal grain growth of ZrO₂ happens. In a method ofproducing the ceramic material of the present invention, a part of CeO₂dissolved into the ZrO₂ grains is used to form the complex oxide of thethird phase during a sintering step. This loss of CeO₂ can be made upwith the addition of TiO₂.

The ZrO₂ based ceramic material of the present invention can be producedin accordance with the following method. That is, a first constituentcorresponding to a composition of 8 to 12 mol % of CeO₂, 0.01 to 0.1 mol% of one of MgO and CaO, and the balance of ZrO₂ is mixed with a secondconstituent for forming Al₂ O₃, to obtain a mixed powder. It ispreferred that the first constituent is provided with a powder having aspecific surface area of 10 to 30 m² /g. The mixed power is molded to agreen compact having a desired shape. Then, the green compact issintered in an oxidative atmosphere at a temperature between 1400° C.and 1600° C. under an atmospheric pressure. A reaction of Ce and one ofMg and Ca supplied from the first constituent with Al supplied from thesecond constituent in the oxidative atmosphere during the sintering formthe third phase of the ceramic material. When the composition of thefirst constituent is used, it is possible to obtain at least 90 vol % oftetragonal ZrO₂ in the first phase. As explained above, a part of CeO₂of the first constituent is used to form the complex oxide of the thirdphase. That is, the part of CeO₂ changes to Ce₂ O₃ at the sinteringtemperature. In other words, a part of Ce changes from tetravalent totrivalent at the sintering temperature. The rest of CeO₂ behaves as thestabilizer of tetragonal ZrO₂. On the other hand, most of MgO or CaO ofthe first constituent reacts with Ce₂ O₃ in the presence of Al₂ O₃grains of the second phase at the sintering temperature to form thecomplex oxide. That is, the trivalent Ce ions react with Al, O, and Mgor Ca at grain boundaries at the sintering temperature to form thecomplex oxide. When a small amount of MgO or CaO remains in the ZrO₂grains of the first phase, it may behave as the stabilizer of tetragonalZrO₂.

In the present method, a combination of the above content of MgO or CaOand the sintering temperature range is important to provide the ceramicmaterial having the third phase of the range of 0.5 to 5 by area %. Whenthe sintering temperature is less than 1400° C. and/or the content ofMgO or CaO is less than 0.01 mol %, a required amount of the complexoxide can not be formed during the sintering step. When the sinteringtemperature is more than 1600° C., and/or the content of MgO or CaO ismore than 0.1 mol %, there are problems that an abnormal crystal growthof the elongated crystals is caused and an excess amount of the thirdphase is formed in the ceramic material. These bring about a decrease inthe mechanical strength of the ceramic material.

When the composition of the first constituent has 0.05 to 4 mol % ofTiO₂, a loss of CeO₂ in the ZrO₂ grains caused by the formation of thecomplex oxide during the sintering step can be made up with TiO₂ capableof behaving as the stabilizer of tetragonal ZrO₂. In addition, it hasalready described that the additive amount of TiO₂ is useful to controla grain growth of the ZrO₂ grains to form the nano-composite structurein the ceramic material.

When the ceramic material has a relative density of 95% or more afterthe sintering step, it is preferred that a hot-isostatic pressing (HIP)treatment is performed to the ceramic material in an oxidativeatmosphere to remove residual pores and further improve the mechanicalproperties. For example, a mixture gas of oxygen gas and a rare gas suchas argon may be used as the oxidative atmosphere. In particular, it ispreferred that the mixture gas contains 5 vol % or more of the oxygengas.

Any one of the following sub-processes 1! to 5! preferably prepares thefirst constituent.

In the sub-process 1!, a zirconia powder containing CeO₂ is mixed with apowder selected from a group of MgCO₃, CaCO₃, MgO, CaO, Mg(OH)₂, andCa(OH)₂, to obtain a first mixed powder. After the first mixed powder isheated to obtain a calcined powder, the calcined powder is milled toobtain the first constituent. It is preferred that the first mixedpowder is heated at a temperature of 800° C. to 1000° C. in the air.

In the sub-process 2!, a mixture solution containing salts of Zr, Ce,and one of Ca and Mg is prepared, and then an alkali solution is addedto the mixture solution to generate a precipitate. After the precipitateis dried and heated to obtain a calcined powder, the calcined powder ismilled to obtain the first constituent. It is preferred that theprecipitate is heated at a temperature of 800° C. to 1000° C. in the airfor several hours.

In the sub-process 3!, a zirconia powder containing CeO₂ and TiO₂ ismixed with a powder selected from a group of MgCO₃, CaCO₃, MgO, CaO,Mg(OH)₂, and Ca(OH)₂, to obtain a first mixed powder. The first mixedpowder is dried and heated to obtain a calcined powder, and then thecalcined powder is milled to obtain the first constituent. It ispreferred that the first mixed powder is heated at a temperature of 800°C. to 1000° C. in the air for several hours.

In the sub-process 4!, a mixture solution containing salts of Zr, Ce,Ti, and one of Ca and Mg is prepared, and then an alkali solution isadded to the mixture solution to generate a precipitate. The precipitateis dried and heated to obtain a calcined powder, and then the calcinedpowder is milled to obtain the first constituent. It is preferred thatthe precipitate is heated at a temperature of 800° C. to 1000° C. in theair for several hours.

In the sub-process 5!, a mixture solution containing salts of Zr, Ce,one of Ca and Mg, and an alkoxide of Ti, is prepared, and then an alkalisolution is added to the mixture solution to generate a precipitate.After the precipitate is dried and heated to obtain a calcined powder,the calcined powder is milled to obtain the first constituent. It ispreferred that the precipitate is heated at a temperature of 800° C. to1000° C. in the air for several hours.

In the sub-processes 1! to 5!, a dry or wet ball-mill may be used forthe powder mixing and/or the milling of the calcined powder. Whenadopting the wet ball-mill, it is preferred to use ethanol, acetone,toluene, or the like as a solvent.

Any one of the following sub-processes 6! and 7! preferably prepares thesecond constituent.

In the sub-process 6!, an aqueous solution of an aluminum salt isprepared, and then an alkali solution such as aqueous ammonia is addedto the aqueous solution to obtain a precipitation. The precipitation isdried and heated at a temperature of about 800° C. in the air forseveral hours to obtain a calcined powder. The calcined powder is milledto obtain an Al₂ O₃ powder as the second constituent.

In the sub-process 7!, an organic solution of an aluminum alkoxide isprepared, and then the aluminum alkoxide is hydrolyzed to obtain aprecipitation. The precipitation is dried and heated at a temperature ofabout 800° C. in the air for several hours to obtain a calcined powder.The calcined powder is milled to obtain an Al₂ O₃ powder as the secondconstituent.

It is also preferred to use as the second constituent a powder of α-Al₂O₃ having an average grain size of 0.5 μm or less or a powder of γAl₂ O₃having a specific surface area of 100 m² /g or more. In particular, whenusing the γ-Al₂ O₃ powder, it is preferred that the followingsub-processes prepare the mixed powder. That is, the first constituentis mixed with the γAl₂ O₃ powder to obtain a first mixed powder. Thefirst mixed powder is dried and heated at a temperature of 1000° C. ormore and less than the sintering temperature to obtain a calcinedpowder, and then the calcined powder is milled to obtain the mixedpowder.

In addition, the following sub-process 8! or 9! preferably prepares themixed powder. That is, in the sub-process 8!, the first constituent ismixed with an aqueous solution of an aluminum salt to obtain a mixturesolution. It is preferred to use the first constituent prepared by anyone of the sub-processes 1! to 5!. An alkali solution such as aqueousammonia is added to the mixture solution to obtain a mixture of thefirst constituent and a precipitation of aluminum hydroxide. After themixture is dried and heated to obtain a calcined powder, the calcinedpowder is milled to obtain the mixed powder. It is preferred that themixture is heated at a temperature of 800° C. in the air for severalhours.

In the sub-process 9!, the first constituent is mixed with an organicsolution of an aluminum alkoxide to obtain a mixture solution. It ispreferred to use the first constituent prepared by any one of thesub-processes 1! to 5!. The aluminum alkoxide of the mixture solution ishydrolyzed to obtain a mixture of the first constituent and aprecipitation of aluminum hydroxide. After the mixture is dried andheated to obtain a calcined powder, the calcined powder is milled toobtain the mixed powder. It is preferred that the mixture is heated at atemperature of 800° C. in the air for several hours.

EXAMPLES 1TO 18

A ZrO₂ based ceramic material of Example 1 was produced by the followingmethod. A ZrO₂ powder containing CeO₂ and having a specific surface areaof 15 m² /g was ball-milled with a MgO powder having an average grainsize of 0.3 μm in the presence of ethanol for 24 hours by the use ofballs made of tetragonal ZrO₂ and a polyethylene vessel. The resultantis then dried to obtain a first mixture as a first constituent. Thecontents of CeO₂ and MgO in the first mixture are 8 mol % and 0.01 mol %relative to ZrO₂, respectively. The first mixture was heated at 950° C.in the air for 3 hours to obtain a calcined powder. The calcined powderwas ball-milled with an α-Al₂ O₃ powder (purity: more than 99.9%) havingan average grain size of 0.2 μm as a second constituent in the presenceof ethanol for 24 hours by the use of the tetragonal ZrO₂ balls and thepolyethylene vessel. The resultant is then dried to obtain a secondmixture. An amount of the α-Al₂ O₃ powder in the second mixture isdetermined such that when all of Al (aluminum) included in the ceramicmaterial is converted to Al₂ O₃, an Al₂ O₃ content in the ceramicmaterial is 30 vol %. The second mixture was molded into a disk having adiameter of 60 mm and a thickness of 5 mm by means of a uni-axis pressmolding and cold isostatic pressing (CIP) treatment. The disk wassintered at 1500° C. in the air for 2 hours under an atmosphericpressure to obtain the ZrO₂ based ceramic material of Example 1. Each ofceramic materials of Examples 2 to 17 was produced in accordance with asubstantially same method as Example 1 except for using a firstconstituent having a different composition of CeO₂ and MgO or CaO, aslisted in Table 1.

Each of the ceramic materials of Examples 1 to 18 was sufficientlydensified by the sintering. By the use of a scanning electron microscope(SEM) and a transmission electron microscope (TEM), it is observed thatthe ceramic material is composed of a ZrO₂ grain phase, α-Al₂ O₃ grainphase, and an elongated crystal phase of a complex oxide formed by areaction of Ce and Mg or Ca supplied from the first mixture with Alsupplied from the α-Al₂ O₃ powder in the oxidative atmosphere at thesintering temperature, as shown in FIGS. 1 and 2. In FIG. 1, arrowsdesignate the elongated crystals dispersed in the ceramic material ofExample 5. As an example, charts of energy dispersive X-ray analysis ofa ZrO₂ grain, α-Al₂ O₃ grain, and an elongated crystal of the ceramicmaterial of Example 5 are shown in FIGS. 3 to 5. FIG. 3 shows that CeO₂is dissolved into the ZrO₂ grain. FIG. 5 shows that the elongatedcrystal contains Ce, Mg and Al. It is presumed that a composition ofthis elongated crystal is CeMgAl₁₁ O₁₉. A content of the elongatedcrystal phase in the ceramic material is presented by an area ratio(area %). That is, the area ratio (area %) of the elongated crystalphase is determined by the following equation;

    Area ratio (Area %)=(t/T)×100

where "T" is a total area of an observation region of a polished andheat-treated surface of the ceramic material observed by SEM, or a totalarea of an observation region of the ceramic material observed by TEM,and "t" is a total area of the elongated crystals exposed on theobservation region of ceramic material. In Examples 1 to 18, the arearatio is within a range of 0.6 to 3.2 area %, an average length of theelongated crystals is within a range of 5.0 to 25.8 μm, and an averageaspect ratio of the elongated crystals is within a range of 5.0 to 19.2.Minimum and maximum lengths of the elongated crystals are 2.0 μm and42.6 μm, respectively. These results are listed in Table 2.

As shown in FIG. 2, average grain sizes of the ZrO₂ grain phase and theα-Al₂ O₃ grain phase of the ceramic material of FIG. 5 are 1.3 μm and0.9 μm, respectively. As listed in Table 1, the average grain size ofthe ZrO₂ grain phase in Examples 1 to 18 is within a range of 0.8 to 1.3μm. The average grain size of the α-Al₂ O₃ grain phase is less than 1 μmin Examples 1 to 18. TEM photographs of the ceramic material of Example5 are shown in FIGS. 6 and 7. These TEM photographs shows that fine Al₂O₃ grains having an average grain size of 1 μm or less are dispersedwithin ZrO₂ grains, and fine ZrO₂ grains are partly dispersed within theelongated crystals of the complex oxide. In another TEM observation, itis confirmed that fine ZrO₂ grains are is partly dispersed withinrelatively large α-Al₂ O₃ grains. A dispersion ratio (W %) of fine α-Al₂O₃ grains dispersed within the ZrO₂ grains is represented by thefollowing equation:

    W(%)=(n/S)×100,

where "S" is the number of the entire Al₂ O₃ grains dispersed at anobservation region in the ceramic material, and "n" is the number of Al₂O₃ grains dispersed within the ZrO₂ grains at the observation region.The numbers "S" and "n" can be counted by the use of TEM and/or SEM. InExamples 1 to 18, the dispersion ratio is within a range of 2.1 to 2.3.

Quantification of tetragonal ZrO₂ in the ZrO₂ grain phase was carriedout by X-ray diffraction analysis. In Tables of this specification, thefollowing classification of ZrO₂ crystal phases is used. That is, when acontent of monoclinic ZrO₂ in the ZrO₂ grain phase is 30 vol % or more,it is designated as "M". When a content of tetragonal ZrO₂ in the ZrO₂grain phase is within a range of 90 vol % to less than 95 vol %, and thebalance is monoclinic ZrO₂, it is designated as "T+M". When the contentof tetragonal ZrO₂ is within a range of 95 vol % or more, and thebalance is monoclinic ZrO₂, it is designated as "T". In addition, whenthe content of tetragonal ZrO₂ is within a range of 90 vol % or more,and the balance is cubic ZrO₂, it is designated as "T+C". Results arelisted in Table 1.

To estimate a mechanical strength of the ceramic material, a 3-pointbending strength was measured according to the test method of JIS(Japanese Industrial Standard) R1601. To prepare specimens of 4 ×3×40 mmfor the 3-point bending test, the ceramic material was cut, ground, andpolished. In addition, fracture toughness of the ceramic material wasmeasured in accordance with IF method. Those results are listed in Table

                  TABLE 1                                                         ______________________________________                                        Starting material     ZrO.sub.2 Based                                                        2nd    Ceramic Material                                        1st constituent  constituent                                                                            Average Grain                                                                            ZrO.sub.2                                (mol %)          (vol %)  Size (μm)                                                                             Crystal                                  CeO.sub.2  MgO    CaO    Al.sub.2 O.sub.3                                                                     ZrO.sub.2                                                                          Al.sub.2 O.sub.3                                                                    Phase                              ______________________________________                                        Example 1                                                                             8      0.01   0    30     1.3  0.9   T + M                            Example 2                                                                             8      0.05   0    30     1.3  0.9   T + M                            Example 3                                                                             8      0.1    0    30     1.3  0.9   T + M                            Example 4                                                                             8      0      0.01 30     1.3  0.9   T + M                            Example 5                                                                             8      0      0.05 30     1.3  0.9   T + M                            Example 6                                                                             8      0      0.1  30     1.3  0.9   T + M                            Example 7                                                                             10     0.01   0    30     0.8  0.4   T                                Example 8                                                                             10     0.05   0    30     0.8  0.4   T                                Example 9                                                                             10     0.1    0    30     0.8  0.4   T                                Example 10                                                                            10     0      0.01 30     0.8  0.4   T                                Example 11                                                                            10     0      0.05 30     0.8  0.4   T                                Example 12                                                                            10     0      0.1  30     0.8  0.4   T                                Example 13                                                                            12     0.01   0    30     0.9  0.4   T                                Example 14                                                                            12     0.05   0    30     0.9  0.4   T                                Example 15                                                                            12     0.1    0    30     0.9  0.4   T                                Example 16                                                                            12     0      0.01 30     0.9  0.4   T                                Example 17                                                                            12     0      0.05 30     0.9  0.4   T                                Example 18                                                                            12     0      0.1  30     0.9  0.4   T                                ______________________________________                                    

                                      TABLE 2                                     __________________________________________________________________________    Bending    Fracture                                                                             Dispersion                                                                          Area Ratio of                                                                         Average Length of                                                                       Average Aspect Ratio                Strength   Toughness                                                                            Ratio of                                                                            Complex Oxide                                                                         Complex oxide                                                                           of Complex Oxide                    (MPa)      (MPa · m.sup.1/2)                                                           Al.sub.2 O.sub.3 (%)                                                                (area %)                                                                               Min.-Max.! (μm)                                                                      Min.-Max.!                         __________________________________________________________________________    Example 1                                                                           580  22.8   2.1   0.7      5.2  2.0-11.6!                                                                         5.3  2.0-9.7!                       Example 2                                                                           630  22.6   2.1   1.7     14.3  5.4-25.6!                                                                         13.8  5.2-18.7!                     Example 3                                                                           670  22.3   2.1   3.1     24.2  8.2-39.6!                                                                         17.6  6.4-22.6!                     Example 4                                                                           550  22.6   2.1   0.7      5.3  2.1-11.4!                                                                         5.2  2.2-9.6!                       Example 5                                                                           600  22.2   2.1   1.6     14.7  6.1-25.9!                                                                         12.7  4.7-18.5!                     Example 6                                                                           660  22.0   2.1   3.0     23.6  7.6-41.6!                                                                         18.1  6.5-23.5!                     Example 7                                                                           780  19.8   2.2   0.7      5.1  2.4-11.7!                                                                         5.3  2.3-9.9!                       Example 8                                                                           850  19.4   2.2   1.7     14.0  5.2-24.3!                                                                         14.1  6.1-19.1!                     Example 9                                                                           940  19.1   2.2   3.2     25.2  7.6-37.6!                                                                         19.2  6.2-22.9!                     Example 10                                                                          750  19.6   2.2   0.6      5.0  2.1-11.2!                                                                          5.4  2.1-10.7!                     Example 11                                                                          810  19.3   2.2   1.6     13.9  4.7-27.2!                                                                         13.5  5.0-18.7!                     Example 12                                                                          880  19.0   2.2   3.1     24.8  6.5-34.6!                                                                         18.4  6.5-23.8!                     Example 13                                                                          790  17.8   2.3   0.7      5.3  2.2-11.5!                                                                         5.0  2.0-8.7!                       Example 14                                                                          880  17.0   2.3   1.7     15.1  5.2-23.4!                                                                         12.9  4.5-17.9!                     Example 15                                                                          960  16.3   2.3   3.1     25.8  8.1-42.6!                                                                         18.4  5.6-23.7!                     Example 16                                                                          770  17.6   2.3   0.7      5.3  2.1-11.8!                                                                         4.8  2.3-7.6!                       Example 17                                                                          830  16.5   2.3   1.5     14.4  4.4-26.8!                                                                         13.3  4.1-18.6!                     Example 18                                                                          900  15.8   2.3   3.1     22.9  7.2-40.1!                                                                         16.3  4.9-24.1!                     __________________________________________________________________________

Examples 19 to 46 and Comparative Examples 1 to 4

A ZrO₂ based ceramic material of Example 19 was produced by thefollowing method. A ZrO₂ powder containing CeO₂ and having a specificsurface area of 15 m² /g, was ball-milled with a MgO powder having anaverage grain size of 0.3 μm and a TiO₂ powder having an average grainsize of 0.3 μm in the presence of ethanol for 24 hours by the use ofballs made of tetragonal ZrO₂ and a polyethylene vessel. The resultantis then dried to obtain a first mixture as a first constituent. Thecontents of CeO₂, TiO₂, and MgO in the first mixture are 8 mol %, 1 mol%, and 0.01 mol % relative to ZrO₂, respectively. The first mixture washeated at 950° C. in the air for 3 hours to obtain a calcined powder.The calcined powder was ball-milled with an α-Al₂ O₃ powder having anaverage grain size of 0.2 μm as a second constituent in the presence ofethanol for 24 hours by the use of the tetragonal ZrO₂ balls and thepolyethylene vessel. The resultant is then dried to obtain a secondmixture. An amount of the α-Al₂ O₃ powder in the second mixture isdetermined such that when all of Al included in the ceramic material isconverted to Al₂ O₃, an Al₂ O₃ content in the ceramic material is 30 vol%. The second mixture was molded into a disk having a diameter of 60 mmand a thickness of 5 mm by means of a uni-axis press molding and coldisostatic pressing (CIP) treatment. The disk was sintered at 1500° C. inthe air for 2 hours under an atmospheric pressure to obtain the ZrO₂based ceramic material of Example 19. Each of ceramic materials ofExamples 20 to 46 and Comparative Examples 1 to 4 was produced inaccordance with a substantially same method as Example 19 except forusing a first constituent having a different composition of CeO₂, TiO₂,and MgO or CaO, as listed in Tables 3 and 4.

Each of the ceramic materials of Examples 19 to 46 was sufficientlydensified by the sintering. By the use of a scanning electron microscope(SEM) and/or a transmission electron microscope (TEM), it is observedthat each of the ceramic materials is composed of a ZrO₂ grain phase,α-Al₂ O₃ grain phase, and an elongated crystal phase of a complex oxideof Ce, Al, and Mg or Ca. An area ratio of the elongated crystal phase inthe ceramic material was measured according to the same manner asExample 1. In Examples 19 to 46, the area ratio is within a range of 0.6to 3.3 area %, an average length of the elongated crystals is within arange of 4.8 to 26.7 μm, and an average aspect ratio of the elongatedcrystals is within a range of 4.6 to 17.6. Minimum and maximum lengthsof the elongated crystals are 2.0 μm and 41.5 μm, respectively. Theseresults are listed in Tables 5 and 6.

The average grain sizes of the ZrO₂ grain phase and the α-Al₂ O₃ grainphase of the ceramic material in Examples 19 to 46 are within a range of0.9 to 4.2 μm, and less than 2 μm, respectively. These results arelisted in Tables 3 and 4. From TEM and SEM observations, it is confirmedthat fine Al₂ O₃ grains having an average grain size of 1 μm or less aredispersed within ZrO₂ grains, and fine ZrO₂ grains are dispersed withinrelatively long crystals of the complex oxide and relatively large α-Al₂O₃ grains. In addition, there is a tendency that as the additive amountof TiO₂ is greater, the average grain size of the ZrO₂ increasesirrespective of the additive amounts of CeO₂ and MgO or CaO. Adispersion ratio (W %) of fine α-Al₂ O₃ grains dispersed within the ZrO₂grains was measured according to the same manner as Example 1. InExamples 19 to 46, the dispersion ratio is within a range of 2.5 to 4.6.

Mechanical strength and fracture toughness of the ceramic material weremeasured in accordance with the same methods as Example 1. Those resultsare listed in Tables 5 and 6. Furthermore, quantification of tetragonalZrO₂ in the ZrO₂ grain phase was carried out by X-ray diffractionanalysis. As listed in Tables 3 and 4, a content of tetragonal ZrO₂ inthe ZrO₂ grain phase in Examples 19 to 22 is within a range of 90 vol %to less than 95 vol %, and the balance is monoclinic ZrO₂. On the otherhand, the content of tetragonal ZrO₂ in the ZrO₂ grain phase in Examples23 to 46 is within a range of 95 vol % or more, and the balance ismonoclinic ZrO₂.

In the ceramic material of Comparative Example 1, it is observed that alarge number of micro-cracks develop during a cooling step from thesintering temperature due to a large tetragonal-to-monoclinic phasetransformation. As a result, the mechanical properties of the ceramicmaterial were not measured. By X-ray diffraction analysis, it wasidentified that a content of monoclinic ZrO₂ in the ZrO₂ grain phasereaches about 80 vol %. As to the ceramic material of ComparativeExample 2, an abnormal grain growth of ZrO₂ up to about 10 μm is oftenobserved in the ceramic material. In addition, it is observed that a lotof residual pores are left within the ZrO₂ grains and at triple pointsof the ZrO₂ grains. By X-ray diffraction analysis, it is confirmed thatthe ZrO₂ grain phase is formed with tetragonal ZrO₂ and cubic ZrO₂. Itis presumed that these structural defects were caused by the addition ofan excess amount of TiO₂. As to the ceramic material of ComparativeExample 3, since a large amount of MgO is included in the firstconstituent, a crystal growth of the elongated crystal phase isenhanced, so that the average mechanical strength of the ceramicmaterial decreases. As to the ceramic material of Comparative Example 4,since neither MgO nor CaO is included in the first constituent, theelongated crystal phase is not formed in the ceramic material, so thatthe fracture toughness lowers.

                                      TABLE 3                                     __________________________________________________________________________    Starting material             ZrO.sub.2 Based Ceramic Material                1st constituent       2nd constituent                                                                       Average Grain Size                              (mol %)               (vol %) (μm)  ZrO.sub.2 Crystal                      CeO.sub.2 MgO CaO TiO.sub.2                                                                         Al.sub.2 O.sub.3                                                                      ZrO.sub.2                                                                         Al.sub.2 O.sub.3                                                                   Phase                                  __________________________________________________________________________    Example 19                                                                          8   0.01                                                                              0   1   30      1.6 1    T + M                                  Example 20                                                                          8   0.1 0   1   30      1.6 1    T + M                                  Example 21                                                                          8   0   0.01                                                                              1   30      1.6 1    T + M                                  Example 22                                                                          8   0   0.1 1   30      1.6 1    T + M                                  Example 23                                                                          8   0.01                                                                              0   4   30      4.2 1.8  T                                      Example 24                                                                          8   0.1 0   4   30      4.2 1.8  T                                      Example 25                                                                          8   0   0.01                                                                              4   30      4.2 1.8  T                                      Example 26                                                                          8   0   0.1 4   30      4.2 1.8  T                                      Example 27                                                                          10  0.01                                                                              0   0.05                                                                              30      0.9 0.4  T                                      Example 28                                                                          10  0.1 0   0.05                                                                              30      0.9 0.4  T                                      Example 29                                                                          10  0   0.01                                                                              0.05                                                                              30      0.9 0.4  T                                      Example 30                                                                          10  0   0.1 0.05                                                                              30      0.9 0.4  T                                      Example 31                                                                          10  0.01                                                                              0   1   30      1.4 0.6  T                                      Example 32                                                                          10  0.1 0   1   30      1.4 0.6  T                                      Example 33                                                                          10  0   0.01                                                                              1   30      1.4 0.6  T                                      Example 34                                                                          10  0   0.1 1   30      1.4 0.6  T                                      Example 35                                                                          10  0.01                                                                              0   4   30      3.8 1.4  T                                      Example 36                                                                          10  0.1 0   4   30      3.8 1.4  T                                      Example 37                                                                          10  0   0.01                                                                              4   30      3.8 1.4  T                                      __________________________________________________________________________

                                      TABLE 4                                     __________________________________________________________________________    Starting material             ZrO.sub.2 Based Ceramic Material                1st constituent       2nd constituent                                                                       Average Grain Size                              (mol %)               (vol %) (μm)  ZrO.sub.2 Crystal                      CeO.sub.2 MgO CaO TiO.sub.2                                                                         Al.sub.2 O.sub.3                                                                      ZrO.sub.2                                                                         Al.sub.2 O.sub.3                                                                   Phase                                  __________________________________________________________________________    Example 38                                                                          10  0   0.1 4   30      3.8 1.4  T                                      Example 39                                                                          12  0.01                                                                              0   0.2 30      1.1 0.5  T                                      Example 40                                                                          12  0.1 0   0.2 30      1.1 0.5  T                                      Example 41                                                                          12  0   0.01                                                                              0.2 30      1.1 0.5  T                                      Example 42                                                                          12  0   0.1 0.2 30      1.1 0.5  T                                      Example 43                                                                          12  0.01                                                                              0   4   30      3.7 1.3  T                                      Example 44                                                                          12  0.1 0   4   30      3.7 1.3  T                                      Example 45                                                                          12  0   0.01                                                                              4   30      3.7 1.3  T                                      Example 46                                                                          12  0   0.1 4   30      3.7 1.3  T                                      Comparative                                                                         6   0.1 0   1   30      --  --   T + M                                  Example 1                                                                     Comparative                                                                         12  0   0.05                                                                              8   30      7.3 3.6  T + C                                  Example 2                                                                     Comparative                                                                         10  2   0   1   30      1.4 0.6  T                                      Example 3                                                                     Comparative                                                                         12  0   0   4   30      4.3 1.5  T                                      Example 4                                                                     __________________________________________________________________________

                                      TABLE 5                                     __________________________________________________________________________    Bending    Fracture                                                                             Dispersion                                                                          Area Ratio of                                                                         Average Length of                                                                       Average Aspect Ratio                Strength   Toughness                                                                            Ratio of                                                                            Complex Oxide                                                                         Complex oxide                                                                           of Complex Oxide                    (MPa)      (MPa · m.sup.1/2)                                                           Al.sub.2 O.sub.3 (%)                                                                (area %)                                                                               Min.-Max.! (μm)                                                                      Min.-Max.!                         __________________________________________________________________________    Example 19                                                                          630  22.4   3.1   0.6      5.2  2.0-11.5!                                                                         5.1  2.0-8.8!                       Example 20                                                                          720  21.8   3.1   3.2     24.2  8.9-37.5!                                                                         17.6  6.2-22.6!                     Example 21                                                                          610  22.2   3.1   0.7      5.2  2.2-11.6!                                                                         5.3  2.2-9.7!                       Example 22                                                                          700  21.6   3.1   3.0     23.8  8.2-39.5!                                                                         17.6  6.4-21.6!                     Example 23                                                                          570  18.2   4.5   0.6      5.1  2.3-12.1!                                                                          4.8  2.3-10.3!                     Example 24                                                                          660  15.1   4.5   3.1     24.6  8.2-38.7!                                                                         17.3  6.2-23.2!                     Example 25                                                                          560  17.7   4.5   0.7      5.3  2.1-11.3!                                                                         5.7  2.0-8.8!                       Example 26                                                                          630  14.4   4.5   3.0     25.1  8.2-36.1!                                                                         17.1  6.8-24.3!                     Example 27                                                                          920  19.6   2.5   0.7      4.8  2.0-10.8!                                                                         5.5  2.2-9.6!                       Example 28                                                                          1160 18.8   2.5   3.2     25.3  8.2-40.6!                                                                         16.8  6.0-21.1!                     Example 29                                                                          870  19.4   2.5   0.6      5.0  2.4-13.2!                                                                         4.7  2.1-9.5!                       Example 30                                                                          1100 18.6   2.5   3.1     26.7  8.2-37.6!                                                                         16.9  6.2-20.6!                     Example 31                                                                          870  18.9   3.4   0.7      4.9  2.6-11.5!                                                                          5.1  2.3-10.1!                     Example 32                                                                          1100 17.7   3.4   3.2     22.9  8.2-39.8!                                                                         17.2  6.5-21.8!                     Example 33                                                                          820  18.5   3.4   0.7      5.4  2.8-14.2!                                                                         5.5  2.0-8.6!                       Example 34                                                                          1060 18.0   3.4   3.1     24.5  8.2-41.5!                                                                         16.4  6.6-22.2!                     Example 35                                                                          830  16.7   4.5   0.7      5.5  2.2-10.7!                                                                         6.0  2.2-8.9!                       Example 36                                                                          1000 14.0   4.5   3.0     23.9  8.2-36.1!                                                                         17.2  6.8-22.6!                     Example 37                                                                          790  16.4   4.5   0.7      6.1  2.5-12.6!                                                                         5.8  2.1-9.1!                       __________________________________________________________________________

                                      TABLE 6                                     __________________________________________________________________________    Bending    Fracture                                                                             Dispersion                                                                          Area Ratio of                                                                         Average Length of                                                                       Average Aspect Ratio                Strength   Toughness                                                                            Ratio of                                                                            Complex Oxide                                                                         Complex oxide                                                                           of Complex Oxide                    (MPa)      (MPa · m.sup.1/2)                                                           Al.sub.2 O.sub.3 (%)                                                                (area %)                                                                               Min.-Max.! (μm)                                                                      Min.-Max.!                         __________________________________________________________________________    Example 38                                                                          920  13.9   4.5   3.1     26.3  8.2-38.5!                                                                         17.6  6.6-23.5!                     Example 39                                                                          910  17.2   2.6   0.7      5.2  2.6-13.5!                                                                         4.6  2.0-7.9!                       Example 40                                                                          1100 15.6   2.6   3.1     23.7  8.2-40.6!                                                                         17.5  6.1-23.3!                     Example 41                                                                          880  16.9   2.6   0.6      5.6  2.2-10.9!                                                                         5.0  2.3-8.3!                       Example 42                                                                          1030 15.0   2.6   3.1     22.9  8.2-37.2!                                                                         16.7  7.1-23.7!                     Example 43                                                                          850  14.2   4.6   0.7      5.3  2.0-11.0!                                                                         5.5  2.1-9.1!                       Example 44                                                                          1020 12.5   4.6   3.3     24.7  8.2-37.8!                                                                         17.3  6.6-22.1!                     Example 45                                                                          820  13.8   4.6   0.7      5.2  2.3-13.4!                                                                         5.8  2.0-7.9!                       Example 46                                                                          950  12.3   4.6   3.2     24.2  8.2-40.9!                                                                         17.4  6.4-22.7!                     Comparative                                                                         --   --     --    --      --        --                                  Example 1                                                                     Comparative                                                                         330  6.3    5.4   1.5     13.1  5.5-12.1!                                                                         12.4  4.6-19.5!                     Example 2                                                                     Comparative                                                                         450  9.0    3.3   7.2      41.0  12.2-75.3!                                                                       21.5  8.9-30.6!                     Example 3                                                                     Comparative                                                                         530  6.6    4.6   --      --        --                                  Example 4                                                                     __________________________________________________________________________

Examples 47 to 51 and Comparative Examples 5 and 6

These Examples and Comparative Examples were produced to investigate aninfluence of an Al₂ O₃ content in the ceramic material to the mechanicalproperties. That is, a ZrO₂ powder containing CeO₂ and having a specificsurface area of 15 m² /g was ball-milled with a MgO powder having anaverage grain size of 0.3 μm and a TiO₂ powder having an average grainsize of 0.3 μm in the presence of ethanol for 24 hours by the use ofballs made of tetragonal ZrO₂ and a polyethylene vessel. The resultantis then dried to obtain a first mixture as a first constituent. Thecontents of CeO₂, TiO₂, and MgO in the first mixture are 10 mol %, 1 mol%, and 0.05 mol % relative to ZrO₂, respectively. The first mixture washeated at 800° C. in the air for 3 hours to obtain a calcined powder.The calcined powder was ball-milled with an α-Al₂ O₃ powder having anaverage grain size of 0.2 μm as a second constituent in the presence ofethanol for 24 hours by the use of the tetragonal ZrO₂ balls and thepolyethylene vessel. The resultant is then dried to obtain a secondmixture. The second mixtures of Examples 47 to 51 and ComparativeExample 6 have different contents of the α-Al₂ O₃ powder, as listed inTable 7. For example, an amount of the α-Al₂ O₃ powder in the secondmixture of Example 49 is determined such that when all of Al included inthe ceramic material is converted to Al₂ O₃, an Al₂ O₃ content in theceramic material is 30 vol %. The second mixture was molded into a diskhaving a diameter of 60 mm and a thickness of 5 mm by means of auni-axis press molding and cold isostatic pressing (CIP) treatment. Thedisk was sintered at 1500° C. in the air for 2 hours under anatmospheric pressure. In Comparative Example 5, the calcined powder ofthe first constituent was molded and sintered without the α-Al₂ O₃powder being used.

Each of the ceramic materials of Examples 47 to 51 was sufficientlydensified by the sintering. By the use of a scanning electron microscope(SEM) and a transmission electron microscope (TEM), it is observed thateach of the ceramic materials is composed of a ZrO₂ grain phase, α-Al₂O₃ grain phase, and an elongated crystal phase of a complex oxide of Ce,Al, and Mg or Ca. An area ratio of the elongated crystal phase in theceramic material was measured according to the same manner as Example 1.In Examples 47 to 51, the area ratio is within a range of 1.6 to 1.8area %, an average length of the elongated crystals is within a range of14.0 to 14.7 μm, and an average aspect ratio of the elongated crystalsis within a range of 12.5 to 14.3. Minimum and maximum lengths of theelongated crystals are 4.6 μm and 28.7 μm, respectively. These resultsare listed in Tables 8.

As listed in Table 7, the average grain sizes of the ZrO₂ grain phaseand the α-Al₂ O₃ grain phase in Examples 47 to 51 are within a range of1.1 to 2.7 μm, and less than 1 μm, respectively. There is a tendencythat as the Al₂ O₃ content in the second mixture increases, a graingrowth of ZrO₂ is prevented. A dispersion ratio (W %) of fine α-Al₂ O₃grains dispersed within the ZrO₂ grains was measured according to thesame manner as Example 1. In Examples 47 to 51, the dispersion ratio iswithin a range of 2.0 to 3.4.

Mechanical strength and fracture toughness of the ceramic materials weremeasured according to the same methods as Example 1. Results are listedin Table 8. Furthermore, quantification of tetragonal ZrO₂ in the ZrO₂grain phase was carried out by X-ray diffraction analysis. As listed inTable 7, a content of tetragonal ZrO₂ in the ZrO₂ grain phase inExamples 47 to 51 and Comparative Examples 5 and 6 is within a range of95 vol % or more, and the balance is monoclinic ZrO₂.

As to Comparative Example 5, the mechanical strength of the ceramicmaterial decreases because of a lack of the Al₂ O₃ grain phase. On theother hand, the ceramic material of Comparative Example 6 shows poormechanical strength and fracture toughness because of an excess amountof Al₂ O₃ grain phase in the ceramic material.

                                      TABLE 7                                     __________________________________________________________________________    Starting material             ZrO.sub.2 Based Ceramic Material                1st constituent       2nd constituent                                                                       Average Grain Size                              (mol %)               (vol %) (μm)  ZrO.sub.2 Crystal                      CeO.sub.2 MgO CaO TiO.sub.2                                                                         Al.sub.2 O.sub.3                                                                      ZrO.sub.2                                                                         Al.sub.2 O.sub.3                                                                   Phase                                  __________________________________________________________________________    Comparative                                                                         10  0.05                                                                              0   1   0       4.1 --   T                                      Example 5                                                                     Example 47                                                                          10  0.05                                                                              0   1   10      2.7 0.5  T                                      Example 48                                                                          10  0.05                                                                              0   1   20      1.7 0.6  T                                      Example 49                                                                          10  0.05                                                                              0   1   30      1.4 0.6  T                                      Example 50                                                                          10  0.05                                                                              0   1   40      1.2 0.7  T                                      Example 51                                                                          10  0.05                                                                              0   1   50      1.1 0.8  T                                      Comparative                                                                         10  0.05                                                                              0   1   60      0.9 1.0  T                                      Example 6                                                                     __________________________________________________________________________

                                      TABLE 8                                     __________________________________________________________________________    Bending    Fracture                                                                             Dispersion                                                                          Area Ratio of                                                                         Average Length of                                                                       Average Aspect Ratio                Strength   Toughness                                                                            Ratio of                                                                            Complex Oxide                                                                         Complex oxide                                                                           of Complex Oxide                    (MPa)      (MPa · m.sup.1/2)                                                           Al.sub.2 O.sub.3 (%)                                                                (area %)                                                                               Min.-Max.! (μm)                                                                      Min.-Max.!                         __________________________________________________________________________    Comparative                                                                         500  21.6   --    --      --        --                                  Example 5                                                                     Example 47                                                                          840  19.8   2.2   1.6     14.0  4.9-25.6!                                                                         12.5  4.4-18.6!                     Example 48                                                                          1000 18.4   2.8   1.7     14.1  5.5-26.3!                                                                         13.5  4.8-19.2!                     Example 49                                                                          1020 17.7   3.4   1.7     14.3  5.0-28.7!                                                                         13.9  4.1-18.8!                     Example 50                                                                          980  17.2   2.6   1.8     14.7  5.2-24.6!                                                                         13.7  4.5-17.7!                     Example 51                                                                          920  15.4   2.0   1.8     14.5  4.6-27.9!                                                                         14.3  4.3-18.3!                     Comparative                                                                         560  11.1   0.9   1.8     14.9  5.5-25.1!                                                                         14.1  4.4-17.8!                     Example 6                                                                     __________________________________________________________________________

Comparative Example 7

A ZrO₂ based ceramic material of Comparative Example 7 was produced inaccordance with a substantially same method as Example 27 except forusing an α-Al₂ O₃ powder having an average grain size of 53 μm in placeof the α-Al₂ O₃ powder used in Example 27.

The ceramic material of Comparative Examples 7 was sufficientlydensified by the sintering. By the use of a scanning electron microscopeand a transmission electron microscope, it is observed that an averagegrain size of an α-Al₂ O₃ grain phase is 5.9 μm, as listed in Table 9,and most of the α-Al₂ O₃ grains are dispersed in grain boundaries of aZrO₂ grain phase. A result of X-ray diffraction analysis shows that theZrO₂ grain phase of this ceramic material is formed with 95 vol % ormore of tetragonal ZrO₂ and the balance of monoclinic ZrO₂. A dispersionratio (W %) of fine α-Al₂ O₃ grains dispersed within the ZrO₂ grains,mechanical strength and fracture toughness of the ceramic material, weremeasured according to the same methods as Example 1. Results are listedin Tables 10.

Due to the large average grain size of the Al₂ O₃ grain phase and adecrease of the dispersion ratio, the mechanical strength and fracturetoughness of the ceramic material of Comparative Example 7 is much lowerthan those of Examples 27.

                                      TABLE 9                                     __________________________________________________________________________    Starting material             ZrO.sub.2 Based Ceramic Material                1st constituent       2nd constituent                                                                       Average Grain Size                              (mol %)               (vol %) (μm)  ZrO.sub.2 Crystal                      CeO.sub.2 MgO CaO TiO.sub.2                                                                         Al.sub.2 O.sub.3                                                                      ZrO.sub.2                                                                         Al.sub.2 O.sub.3                                                                   Phase                                  __________________________________________________________________________    Example 27                                                                          10  0.01                                                                              0   0.05                                                                              30      0.9 0.4  T                                      Comparative                                                                         10  0.01                                                                              0   0.05                                                                              30      1.1 5.9  T                                      Example 7                                                                     __________________________________________________________________________

                                      TABLE 10                                    __________________________________________________________________________    Bending    Fracture                                                                             Dispersion                                                                          Area Ratio of                                                                         Average Length of                                                                       Average Aspect Ratio                Strength   Toughness                                                                            Ratio of                                                                            Complex Oxide                                                                         Complex oxide                                                                           of Complex Oxide                    (MPa)      (MPa · m.sup.1/2)                                                           Al.sub.2 O.sub.3 (%)                                                                (area %)                                                                               Min.-Max.! (μm)                                                                      Min.-Max.!                         __________________________________________________________________________    Example 27                                                                          920  19.6   2.5   0.7     4.8  2.0-10.8!                                                                          5.5  2.2-9.6!                       Comparative                                                                         370  10.6   0.3   0.2     6.5  3.6-8.4!                                                                           1.6  1-3.5!                         Example 7                                                                     __________________________________________________________________________

EXAMPLES 52

A ZrO₂ based ceramic material of Example 52 was produced in accordancewith a substantially same method as Example 27 except that the sinteringtemperature is 1450° C., and a HIP (Hot Isostatic Pressing) treatmentwas performed after the sintering step at a temperature of 1350° C. foran 1 hour under a pressure of 150 MPa of a mixture gas of argon andoxygen (argon/oxygen =90/10).

The ceramic material of Example 52 was sufficiently densified by the HIPtreatment. By the use of a scanning electron microscope and transmissionelectron microscope, it is observed that an average grain size of a ZrO₂grain phase is slightly small than that of Example 27, as listed inTable 11, and some of fine Al₂ O₃ grains are dispersed within the ZrO₂grains. In addition, it is observed that fine ZrO₂ grains are dispersedwithin relatively long crystals of a complex oxide of Al, Ce and Mg, andrelatively large α-Al₂ O₃ grains. A result of X-ray diffraction analysisshows that the ZrO₂ grain phase of this ceramic material is formed with95 vol % or more of tetragonal ZrO₂ and the balance of monoclinic ZrO₂.A dispersion ratio (W %) of fine αAl₂ O₃ grains dispersed within theZrO₂ grains, mechanical strength and fracture toughness of the ceramicmaterial, were measured in accordance with the same methods asExample 1. Results are listed in Table 12. The results show that the HIPtreatment is useful to improve the mechanical strength.

                                      TABLE 11                                    __________________________________________________________________________    Starting material             Composite Ceramic Material                      1st constituent       2nd constituent                                                                       Average Grain Size                              (mol %)               (vol %) (μm)  ZrO.sub.2 Crystal                      CeO.sub.2 MgO CaO TiO.sub.2                                                                         Al.sub.2 O.sub.3                                                                      ZrO.sub.2                                                                         Al.sub.2 O.sub.3                                                                   Phase                                  __________________________________________________________________________    Example 27                                                                          10  0.01                                                                              0   0.05                                                                              30      0.9 0.4  T                                      Example 52                                                                          10  0.01                                                                              0   0.05                                                                              30      0.8 0.4  T                                      __________________________________________________________________________

                                      TABLE 12                                    __________________________________________________________________________    Bending    Fracture                                                                             Dispersion                                                                          Area Ratio of                                                                         Average Length of                                                                       Average Aspect Ratio                Strength   Toughness                                                                            Ratio of                                                                            Complex Oxide                                                                         Complex oxide                                                                           of Complex Oxide                    (MPa)      (MPa · m.sup.1/2)                                                           Al.sub.2 O.sub.3 (%)                                                                (area %)                                                                               Min.-Max.! (μm)                                                                      Min.-Max.!                         __________________________________________________________________________    Example 27                                                                           920 19.6   2.5   0.7     4.8  2.0-10.8!                                                                          5.5  2.2-9.6!                       Example 52                                                                          1150 19.1   2.4   0.8     4.2  2.1-7.6!                                                                           5.1  2.2-7.7!                       __________________________________________________________________________

EXAMPLES 53 TO 60

ZrO₂ based ceramic materials of Examples 53 and 54 were produced inaccordance with a substantially same method as Example 1 except that aMgCO₃ powder having an average grain size of 0.3 μm was used in place ofthe MgO powder. As listed in Table 13, an amount of the MgCO₃ powderused in Example 53 is determined such that when MgCO₃ in a first mixtureis converted to MgO, a MgO content in the first mixture is 0.01 mol %relative to ZrO₂. Similarly, an amount of the MgCO₃ powder used inExample 54 is determined such that the MgO content is 0.1 mol % relativeto ZrO₂.

ZrO₂ based ceramic materials of Examples 55 and 56 were produced inaccordance with a substantially same method as Example 1 except that aCaCO₃ powder having an average grain size of 0.3 μm was used in place ofthe MgO powder. As listed in Table 13, an amount of the CaCO₃ powderused in Example 55 is determined such that when CaCO₃ in a first mixtureis converted to CaO, a CaO content in the first mixture is 0.01 mol %relative to ZrO₂. Similarly, an amount of the CaCO₃ powder used inExample 56 is determined such that the CaO content is 0.1 mol % relativeto ZrO₂.

ZrO₂ based ceramic materials of Examples 57 and 58 were produced inaccordance with a substantially same method as Example 1 except that aMg(OH)₂ powder having an average grain size of 0.3 μm was used in placeof the MgO powder. As listed in Table 13, an amount of the Mg(OH)₂powder used in Example 57 is determined such that when Mg(OH)₂ in afirst mixture is converted to MgO, a MgO content in the first mixture is0.01 mol % relative to ZrO₂. Similarly, the amount of the Mg(OH)₂ powderused in Example 58 is determined such that the MgO content is 0.1 mol %relative to ZrO₂.

ZrO₂ based ceramic materials of Examples 59 and 60 were produced inaccordance with a substantially same method as Example 1 except that aCa(OH)₂ powder having an average grain size of 0.3 μm was used in placeof the MgO powder. As listed in Table 13, an amount of the Ca(OH)₂powder used in Example 59 is determined such that when Ca(OH)₂ in afirst mixture is converted to CaO, a CaO content in the first mixture is0.01 mol % relative to ZrO₂. Similarly, an amount of the Ca(OH)₂ powderused in Example 60 is determined such that the CaO content is 0.1 mol %relative to ZrO₂.

Each of the ceramic materials of Examples 53 to 60 was sufficientlydensified by the sintering. By the use of a scanning electron microscopeand a transmission electron microscope, it is observed that theseceramic materials has a common micro-structure composed of a ZrO₂ grainphase, α-Al₂ O₃ grain phase, and an elongated crystal phase of a complexoxide of Ce, Al, and Mg or Ca. An area ratio of the elongated crystalphase in the ceramic material was measured according to the same manneras Example 1. In Examples 53 to 60, the area ratio of the elongatedcrystal phase is within a range of 0.6 to 3.2 area %, an average lengthof the elongated crystals is within a range of 4.9 to 24.7 μm, and anaverage aspect ratio of the elongated crystals is within a range of 4.8to 17.8. Minimum and maximum lengths of the elongated crystals are 2.0μm and 41.3 μm, respectively. These results are listed in Tables 14.

In addition, it is observed that fine Al₂ O₃ grains are dispersed withinthe ZrO₂ grains, and fine ZrO₂ grains are dispersed within relativelylong crystals of the complex oxide and relatively large α-Al₂ O₃ grains.Results of X-ray diffraction analysis show that a content of tetragonalZrO₂ in the ZrO₂ grain phase in Examples 53 to 60 is within a range of95 vol % or more, and the balance is monoclinic ZrO₂. A dispersion ratio(W %) of fine α-Al₂ O₃ grains dispersed within the ZrO₂ grains,mechanical strength and fracture toughness of the ceramic material, weremeasured according to the same methods as Example 1. Results are listedon Table 14.

                                      TABLE 13                                    __________________________________________________________________________    Starting material                                                             1st constituent                        ZrO.sub.2 Based Ceramic Material       (mol %)                        2nd constituent                                                                       Average Grain Size                     MgO)        CaO   MgO    CaO   (vol %) (μm)  ZrO.sub.2 Crystal             (MgCO.sub.3)                                                                              (CaCO.sub.3)                                                                        (Mg(OH).sub.2)                                                                       (Ca(OH).sub.2)                                                                      Al.sub.2 O.sub.3                                                                      ZrO.sub.2                                                                         Al.sub.2 O.sub.3                                                                   Phase                         __________________________________________________________________________    Example 53                                                                          0.01  0     0      0     30      1.4 0.6  T                             Example 54                                                                          0.1   0     0      0     30      1.4 0.6  T                             Example 55                                                                          0     0.01  0      0     30      1.4 0.6  T                             Example 56                                                                          0     0.1   0      0     30      1.4 0.6  T                             Example 57                                                                          0     0     0.01   0     30      1.6 0.7  T                             Example 58                                                                          0     0     0.1    0     30      1.6 0.7  T                             Example 59                                                                          0     0     0      0.01  30      1.6 0.7  T                             Example 60                                                                          0     0     0      0.1   30      1.6 0.7  T                             __________________________________________________________________________

                                      TABLE 14                                    __________________________________________________________________________    Bending    Fracture                                                                             Dispersion                                                                          Area Ratio of                                                                         Average Length of                                                                       Average Aspect Ratio                Strength   Toughness                                                                            Ratio of                                                                            Complex Oxide                                                                         Complex oxide                                                                           of Complex Oxide                    (MPa)      (MPa · m.sup.1/2)                                                           Al.sub.2 O.sub.3 (%)                                                                (area %)                                                                               Min.-Max.! (μm)                                                                      Min.-Max.!                         __________________________________________________________________________    Example 53                                                                          900  18.1   3.4   0.6      5.1  2.0-10.6!                                                                         5.3  2.2-8.6!                       Example 54                                                                          1130 17.0   3.4   2.9     24.2  8.0-37.6!                                                                         17.4  7.4-21.1!                     Example 55                                                                          880  17.8   3.4   0.7      5.3  2.1-11.8!                                                                         5.2  2.3-9.5!                       Example 56                                                                          1100 16.8   3.4   3.1     23.6  7.5-38.6!                                                                         17.0  7.3-22.3!                     Example 57                                                                          920  17.3   3.5   0.7      4.9  2.2-11.3!                                                                         4.8  2.4-8.1!                       Example 58                                                                          1140 16.2   3.5   3.0     24.7  8.2-40.6!                                                                         17.6  6.9-23.4!                     Example 59                                                                          900  17.0   3.5   0.7      5.5  2.3-12.1!                                                                         5.5  2.1-9.2!                       Example 60                                                                          1120 15.9   3.5   3.2     25.1  7.6-41.3!                                                                         17.8  7.8-23.1!                     __________________________________________________________________________

EXAMPLES 61 TO 63

In Example 61, a first aqueous solution of zirconium oxychloride(ZrOCl₂.8H₂ O) was hydrolyzed by adding aqueous ammonia thereto toobtain a sol solution of ZrO₂. The sol solution was mixed with a secondaqueous solution of cerium chloride (CeCl₃.7H₂ O), a third aqueoussolution of titanium chloride (TiCl₄), and a fourth aqueous solution ofmagnesium chloride (MgCl₂), while agitating a resultant mixture. Themixture was dropped into aqueous ammonia, while agitating the aqueousammonia, to thereby obtain a precipitate. After the precipitate waswashed with water and dried, it was heated at 950° C. in the air for 3hours to obtain a calcined powder as a first constituent. Amounts andconcentrations of the second to fourth aqueous solutions in the mixtureare determined such that the calcined powder contains 10 mol % of CeO₂,1 mol % of TiO₂, and 0.1 mol % of MgO relative to ZrO₂, as listed inTable 15. The calcined powder was ball-milled with a γ-Al₂ O₃ powderhaving a specific surface area of 300 m² /g as a second constituent inthe presence of ethanol for 24 hours by the use of balls made oftetragonal ZrO₂ and a polyethylene vessel. The resultant is then driedto obtain a mixed powder of Example 61. An amount of the γ-Al₂ O₃ powderin the mixed powder is determined such that when all of Al included in aZrO₂ based ceramic material of Example 61 is converted to Al₂ O₃, an Al₂O₃ content in the ceramic material is 30 vol %.

In Example 62, the calcined powder prepared in Example 61 is mixed witha hydrochloride solution of aluminum chloride (AlCl₃), while agitating aresultant, to thereby obtain a first mixture. The first mixture washydrolyzed by an aqueous solution of sodium hydroxide (NaOH) to obtain asecond mixture of the calcined power and a precipitate of aluminumhydroxide. After the second mixture was washed with water and dried, itwas heated at 1000° C. in the air for 3 hours to change aluminumhydroxide to Al₂ O₃ and obtain a mixed powder of Example 62. An amountand a concentration of the hydrochloride solution of AlCl₃ in the firstmixture are determined such that when all of Al included in a ZrO₂ basedceramic material of Example 62 is converted to Al₂ O₃, an Al₂ O₃ contentin the ceramic material is 30 vol %.

In Example 63, a first aqueous solution of zirconium oxychloride(ZrOCl₂.8H₂ O) was hydrolyzed by adding aqueous ammonia thereto toobtain a sol solution of ZrO₂. The sol solution was mixed with a secondaqueous solution of cerium chloride (CeCl₃.7H₂ O), a third aqueoussolution of magnesium chloride (MgCl₂), and a first isopropanol solutionof titanium isopropoxide Ti(iOC₃ H₇)₄ !, while agitating a resultant, tothereby obtain a first mixture. The first mixture was dropped intoaqueous ammonia, while agitating the aqueous ammonia, to thereby obtaina first precipitate. After the first precipitate was washed with waterand dried, it was heated at 850° C. in the air for 3 hours to obtain acalcined powder as a first constituent. Amounts and concentrations ofthe second and third aqueous solutions and the first isopropanolsolution in the first mixture are determined such that the calcinedpowder contains 10 mol % of CeO₂, 1 mol % of TiO₂, and 0.1 mol % of MgOrelative to ZrO₂, as listed in Table 15. The calcined powder was mixedwith a second isopropanol solution of aluminum isopropoxide Al(iOC₃ H₇)₃! to obtain a mixed solution. The mixed solution was hydrolyzed toobtain a second mixture of the calcined power and a precipitate. Afterthe second mixture was washed with water and dried, it was heated at1000 ° C. in the air for 3 hours to obtain a mixed powder of Example 63.An amount and a concentration of the second isopropanol solution in themixed solution are determined such that when all of Al included in aZrO₂ based ceramic material of Example 63 is converted to Al₂ O₃, an Al₂O₃ content in the ceramic material is 30 vol %.

The mixed powder formed in each of Examples 61 to 63 was molded into adisk having a diameter of 60 mm and a thickness of 5 mm by means of auni-axis press molding and a cold isostatic pressing (CIP) treatment.The disk was sintered at 1500° C. in the air under an atmosphericpressure for 2 hours to obtain the ZrO₂ based ceramic material.

Each of the ceramic materials of Examples 61 to 63 was sufficientlydensified by the sintering. By the use of a scanning electron microscopeand a transmission electron microscope, it is observed that theseceramic materials has a common micro-structure composed of a ZrO₂ grainphase, α-Al₂ O₃ grain phase, and an elongated crystal phase of a complexoxide of Ce, Al, and Mg. An area ratio of the elongated crystal phase inthe ceramic material was measured according to the same manner asExample 1. In Examples 61 to 63, the area ratio is within a range of03.1 to 3.3 area %, an average length of the elongated crystals iswithin a range of 23.8 to 24.5 μm, and an average aspect ratio of theelongated crystals is within a range of 17.1 to 17.4. Minimum andmaximum lengths of the elongated crystals are 8.0 μm and 41.5 μm,respectively. These results are listed in Tables 16.

In addition, it is observed that a relatively large amount of fine Al₂O₃ grains are dispersed within the ZrO₂ grains. From X-ray diffractionanalysis, it is confirmed that a content of tetragonal ZrO₂ in the ZrO₂grain phase in Examples 61 to 63 is within a range of 95 vol % or more,and the balance is monoclinic ZrO₂. In Example 61, it is also confirmedthat the γ-Al₂ O₃ powder is completely converted to α-Al₂ O₃. Adispersion ratio (W %) of fine oc-Al₂ O₃ grains dispersed within theZrO₂ grains, mechanical strength and fracture toughness of the ceramicmaterials, were measured in accordance with the same methods asExample 1. Results are listed on Table 16.

                                      TABLE 15                                    __________________________________________________________________________    Starting material             ZrO.sub.2 Based Ceramic Material                1st constituent       2nd constituent                                                                       Average Grain Size                              (mol %)               (vol %) (μm)  ZrO.sub.2 Crystal                      CeO.sub.2 MgO CaO TiO.sub.2                                                                         Al.sub.2 O.sub.3                                                                      ZrO.sub.2                                                                         Al.sub.2 O.sub.3                                                                   Phase                                  __________________________________________________________________________    Example 61                                                                          10  0.1 0   1   30      1.4 0.6  T                                      Example 62                                                                          10  0.1 0   1   30      1.4 0.6  T                                      Example 63                                                                          10  0.1 0   1   30      1.4 0.6  T                                      __________________________________________________________________________

                                      TABLE 16                                    __________________________________________________________________________    Bending    Fracture                                                                             Dispersion                                                                          Area Ratio of                                                                         Average Length of                                                                       Average Aspect Ratio                Strength   Toughness                                                                            Ratio of                                                                            Complex Oxide                                                                         Complex oxide                                                                           of Complex Oxide                    (MPa)      (MPa · m.sup.1/2)                                                           Al.sub.2 O.sub.3 (%)                                                                (area %)                                                                               Min.-Max.! (μm)                                                                      Min.-Max.!                         __________________________________________________________________________    Example 61                                                                          1160 17.5   3.6   3.1     23.8  8.3-40.6!                                                                         17.3  7.0-22.8!                     Example 62                                                                          1170 17.6   3.7   3.3     24.5  9.7-39.9!                                                                         17.1  7.3-23.1!                     Example 63                                                                          1150 17.7   3.6   3.2     24.4  8.0-41.5!                                                                         17.4  7.2-23.5!                     __________________________________________________________________________

Examples 64-65 and Comparative Examples 8-9

ZrO₂ based ceramic materials of these Examples and Comparative Examplesare produced in accordance with a substantially same method as Example28 except for adopting different sintering temperatures, as listed inTable 17.

In Examples 64 and 65, each of the ceramic materials was sufficientlydensified by the sintering. By the use of a scanning electron microscopeand a transmission electron microscope, it is observed that theseceramic materials has a common micro-structure composed of a ZrO₂ grainphase, α-Al₂ O₃ grain phase, and an elongated crystal phase of a complexoxide of Ce, Al, and Mg. As listed in Table 17, as the sinteringtemperature is higher from 1400° C. toward 1600° C., there is a tendencyof increasing average grain sizes of the ZrO₂ grain phase and α-Al₂ O₃grain phase, an area ratio of the elongated crystal phase in the ceramicmaterial, and a dispersion ratio (W %) of fine α-Al₂ O₃ grains dispersedwithin the ZrO₂ grains. In Examples 28, 64 and 65, the ceramic materialof Example 28 sintered at 1500° C. shows the maximum mechanical strengthand fracture toughness, i.e., 1160 MPa and 18.8 MPa.m_(1/2). From X-raydiffraction analysis, it is confirmed that a content of tetragonal ZrO₂in the ZrO₂ grain phase in Examples 64 and 65 is within a range of 95vol % or more, and the balance is monoclinic ZrO₂.

In Comparative Example 8, the ceramic material was not densely sinteredat the low sintering temperature of 1300° C., so that both of themechanical strength and fracture toughness remarkably decrease, aslisted in Table 18. In addition, the elongated crystal phase was notformed in the ceramic material. On man the other hand, the ceramicmaterial of Comparative Example 9 was densely sintered at the highsintering temperature of 1700° C. However, since a crystal growth of theelongated crystal phase and grain growths of the ZrO₂ grain phase andα-Al₂ O₃ grain phase excessively proceed at the high sinteringtemperature, the ceramic material shows a poor bending strength, aslisted in Table 18.

From these results of Examples 1 to 65, it would be understood that theZrO₂ based ceramic materials included in the present invention couldprovide excellent mechanical properties, and particularly great fracturetoughness.

                                      TABLE 17                                    __________________________________________________________________________    Starting material                   ZrO.sub.2 Based Ceramic Material          1st constituent       2nd constituent                                                                       Sintering                                                                           Average Grain Size                        (mol %)               (vol %) Temp. (μm)  ZrO.sub.2 Crystal                CeO.sub.2 MgO CaO TiO.sub.2                                                                         Al.sub.2 O.sub.3                                                                      (°C.)                                                                        ZrO.sub.2                                                                         Al.sub.2 O.sub.3                                                                   Phase                            __________________________________________________________________________    Comparative                                                                         10  0.1 0   0.05                                                                              30      1300  0.3 0.2  T                                Example 8                                                                     Example 64                                                                          10  0.1 0   0.05                                                                              30      1400  0.6 0.3  T                                Example 28                                                                          10  0.1 0   0.05                                                                              30      1500  0.9 0.4  T                                Example 65                                                                          10  0.1 0   0.05                                                                              30      1600  1.5 0.8  T                                Comparative                                                                         10  0.1 0   0.05                                                                              30      1700  3.5 3.1  T                                Example 9                                                                     __________________________________________________________________________

                                      TABLE 18                                    __________________________________________________________________________    Bending    Fracture                                                                             Dispersion                                                                          Area Ratio of                                                                         Average Length of                                                                       Average Aspect Ratio                Strength   Toughness                                                                            Ratio of                                                                            Complex Oxide                                                                         Complex oxide                                                                           of Complex Oxide                    (MPa)      (MPa · m.sup.1/2)                                                           Al.sub.2 O.sub.3 (%)                                                                (area %)                                                                               Min.-Max.! (μm)                                                                      Min.-Max.!                         __________________________________________________________________________    Comparative                                                                         320  9.5    0.5   --      --        --                                  Example 8                                                                     Example 64                                                                          670  13.3   2.1   0.9     17.2  6.9-27.3!                                                                         15.1  5.9-18.1!                     Example 28                                                                          1160 18.8   2.5   3.2     25.3  8.2-40.6!                                                                         16.8  6.0-21.1!                     Example 65                                                                          700  18.6   2.8   5.0     34.6  9.5-50.0!                                                                         19.5  7.9-25.0!                     Comparative                                                                         440  16.7   0.2   6.3      51.2  16.3-82.3!                                                                       23.6  8.6-28.3!                     Example 9                                                                     __________________________________________________________________________

What is claimed is:
 1. A ZrO₂ based ceramic material comprising:a firstphase of ZrO₂ grains containing CeO₂ as a stabilizer and having anaverage grain size of 5 μm or less, at least 90 vol % of said firstphase composed of tetragonal ZrO₂ ; a second phase of Al₂ O₃ grainshaving an average grain size of 2 μm or less; a third phase of elongatedcrystals of a complex oxide of Al, Ce, and one of Mg and Ca; an aluminumcontent in said ceramic material being determined such that whenaluminum of said complex oxide is converted to Al₂ O₃, a total amount ofAl₂ O₃ in said ceramic material is within a range of 0.5 to 50 vol %; acontent of said third phase in said ceramic material being determinedwithin a range of 0.5 to 5 by area %.
 2. The ZrO₂ based ceramic materialas set forth in claim 1, wherein Al₂ O₃ grains having an average grainsize of 1 μm or less of said second phase are dispersed within said ZrO₂grains at a dispersion ratio of at least 2%, said dispersion ratio beingdefined as a ratio of the number of said Al₂ O₃ grains dispersed withinsaid ZrO₂ grains relative to the entire Al₂ O₃ grains dispersed in saidceramic material.
 3. The ZrO₂ based ceramic material as set forth inclaim 2, wherein said first phase contains 0.05 to 4 mol % of TiO₂. 4.The ZrO₂ based ceramic material as set forth in claim 1, wherein saidelongated crystals have an average length of 2 to 50 μm with a maximumlength up to 70 μm.
 5. The ZrO₂ based ceramic material as set forth inclaim 4, wherein an average aspect ratio of said elongated crystals iswithin a range of 2 to 25, said aspect ratio being defined as a ratio oflength to width of said elongated crystals.
 6. The ZrO₂ based ceramicmaterial as set forth in claim 1, wherein ZrO2 grains having an averagegrain size of 1 μm or less of said first phase are dispersed within saidAl₂ O₃ grains.
 7. The ZrO₂ based ceramic material as set forth in claim1, wherein ZrO₂ grains having an average grain size of 1 μm or less ofsaid first phase are dispersed within said elongated crystals of saidthird phase.
 8. A method of producing said ZrO₂ based ceramic materialof claim 1 comprising the steps of:mixing a first constituentcorresponding to a composition of 8 to 12 mol % of CeO₂, 0.01 to 0.1 mol% of one of MgO and CaO, and the balance of ZrO₂ with a secondconstituent for forming Al₂ O₃, to obtain a mixed powder; molding saidmixed power to a green compact having a desired shape; and sinteringsaid green compact in an oxidative atmosphere at a temperature between1400° C. and 1600° C. under an atmospheric pressure, said third phase ofsaid ceramic material being formed by a reaction of Ce and one of Mg andCa supplied from said first constituent with Al supplied from saidsecond constituent in said oxidative atmosphere during the sintering. 9.The method as set forth in claim 8, wherein said composition of saidfirst constituent contains 0.05 to 4 mol % of TiO₂.
 10. The method asset forth in claim 9, wherein said first constituent is prepared by thesteps of:mixing a ZrO₂ powder containing CeO₂ and TiO₂ with a powderselected from a group of MgCO₃, CaCO₃, MgO, CaO, Mg(OH)₂, and Ca(OH)₂,to obtain a first mixed powder; heating said first mixed powder toobtain a calcined powder; and milling said calcined powder.
 11. Themethod as set forth in claim 9, wherein said first constituent isprepared by the steps of:forming a mixture solution containing salts ofZr, Ce, Ti, and one of Ca and Mg; adding an alkali solution to saidmixture solution to generate a precipitate; drying and heating saidprecipitate to obtain a calcined powder; and milling said calcinedpowder.
 12. The method as set forth in claim 9, wherein said firstconstituent is prepared by the steps of:forming a mixture solutioncontaining salts of Zr, Ce, one of Ca and Mg, and an alkoxide of Ti;adding an alkali solution to said mixture solution to generate aprecipitate; drying and heating said precipitate to obtain a calcinedpowder; and milling said calcined powder.
 13. The method as set forth inclaim 8, wherein said first constituent is prepared by the stepsof:mixing a ZrO₂ powder containing CeO₂ with a powder selected from agroup of MgCO₃, CaCO₃, MgO, CaO, Mg(OH)₂, and Ca(OH)₂, to obtain a firstmixed powder; heating said first mixed powder to obtain a calcinedpowder; and milling said calcined powder.
 14. The method as set forth inclaim 8, wherein said first constituent is prepared by the stepsof:forming a mixture solution containing salts of Zr, Ce, and one of Caand Mg; adding an alkali solution to said mixture solution to generate aprecipitate; drying and heating said precipitate to obtain a calcinedpowder; and milling said calcined powder.
 15. The method as set forth inclaim 8, wherein said mixed powder is prepared by the steps of:mixingsaid first constituent with an aqueous solution of an aluminum salt toobtain a mixture solution; adding an alkali solution to said mixturesolution to obtain a mixture of said first constituent and aprecipitation of aluminum hydroxide; drying and heating said mixture toobtain a calcined powder; and milling said calcined powder.
 16. Themethod as set forth in claim 8, wherein said mixed powder is prepared bythe steps of:mixing said first constituent with an organic solution ofan aluminum alkoxide to obtain a mixture solution; hydrolyzing saidaluminum alkoxide in said mixture solution to obtain a mixture of saidfirst constituent and a precipitation; drying and heating said mixtureto obtain a calcined powder; and milling said calcined powder.
 17. Themethod as set forth in claim 8, wherein said second constituent is apowder of α-Al₂ O₃ having an average grain size of 0.5 μm or less. 18.The method as set forth in claim 8, wherein said second constituent is apowder of γ-Al₂ O₃ having a specific surface area of 100 m² /g or more.19. The method as set forth in claim 18, wherein said mixed powder isprepared by the steps of:mixing said first constituent with said γ-Al₂O₃ powder to obtain a first mixed powder; heating said first mixedpowder at a temperature of 1000° C. or more and less than said sinteringtemperature to obtain a calcined powder; and milling said calcinedpowder.
 20. The method as set forth in claim 8, further comprising thestep of performing a hot isostatic pressing (HIP) treatment to saidceramic material in an oxidative atmosphere after said sintering step.21. The method as set forth in claim 8, wherein said first constituentis provided with a powder having a specific surface of 10 to 30 m² /g.